An X-ray computed tomography (CT) apparatus comprises an X-ray tube radiating fan-beam X-rays, and an X-ray detector comprising a large number of aligned devices for detecting fan-beam X-rays, detecting fan-beam X-rays with different angles in each cross section, and analyzing the resultant X-ray absorption data by a computer to calculate X-ray absorbance at each position in each cross section, thereby forming an image in each cross section.
An X-ray detector practically used is a combination of a scintillator of a single-crystal CdWO4 ceramic, a ceramic having a composition of (Y, Gd)2O3:Eu, Pr, a ceramic having a composition of Gd2O2S:Pr, Ce, F (hereinafter referred to as “GOS:Pr”), etc., and a silicon photodiode. In the X-ray detector, the scintillator absorbs X-rays to emit light, which is detected by the silicon photodiode. The scintillator emits light having a wavelength corresponding to an energy level of a light-emitting element (for example, Pr in Gd2O2S:Pr, Ce, F) in the matrix (for example, Gd2O2S in Gd2O2S:Pr, Ce, F). When this wavelength is 500 nm or more corresponding to the visible light, the silicon photodiode has good detection efficiency, providing the X-ray detector with high sensitivity.
The scintillator is required to have such properties as high material uniformity, small unevenness of X-ray characteristics, little deterioration by radiation, small changes of light emission characteristics by environment changes such as temperature variations, etc., good machinability, little deterioration by working, no moisture absorption and no deliquescence, chemical stability, etc.
In the X-ray CT apparatus, the X-ray detection device should be small for improved resolution, and the scanning time should be short to reduce exposure to X-rays and the influence of the body motion. As a result, the amount of X-rays received by each X-ray detection device decreases, so that the X-ray detection device should have high emission efficiency (large emission intensity). Further, to improve the time resolution of the X-ray detection device, emission (afterglow) after the termination of the irradiation of X-rays should be as short as possible. The afterglow is expressed by a ratio of emission intensity after a predetermined period of time from the termination of the irradiation of X-rays to emission intensity during the irradiation of X-rays.
With respect to various scintillators put into practical use at present, their emission intensities and afterglow after 3 ms and 300 ms are shown in Table 1. Using a W target as an X-ray target with no X-ray filter, the emission intensity and the afterglow are measured by irradiating X-rays (containing both soft X-rays and hard X-rays) obtained at tube voltage of 120 kV and tube current of 20 mA to an X-ray detection device comprising a scintillator and a silicon photodiode (S2281 available from Hamamatsu Photonics K. K.). The emission intensity is a relative value (%) when the emission intensity of GOS:Pr, Ce, F is 100%. The afterglow is a relative value (ppm) to emission intensity during the irradiation of X-rays.
TABLE 1Afterglow Emission(ppm)CrystalDensityIntensityAfter AfterCompositionStructure(g/cm3)(%)3 ms300 msCdWO4Single Crystal7.99565005Gd2O2S:Pr, Ce, FPolycrystalline7.2810030010(Y, Gd)2O3:Eu, PrPolycrystalline5.9210030,00010Gd3Ga5O12:Cr, CePolycrystalline7.09721,00050Gd3Al3Ga2O12:CePolycrystalline6.46951,00050
Among the above scintillators, a polycrystalline ceramic of Gd3Al3Ga2O12:Ce (hereinafter referred to as “GGAG:Ce”) comprising gadolinium oxide, gallium oxide, aluminum oxide and cerium oxide as main components and having a garnet structure has a large emission intensity, because light emission occurs by the transition of Ce3+, an light-emitting element, from a 5 d level to a 4 f level. It also has a small decay time constant, which is a time period until emission intensity becomes 1/e of the emission intensity during the irradiation of X-rays after the termination of the irradiation of X-rays.
JP 2001-4753 A discloses an phosphorescent oxide as a ceramic scintillator, which is an oxide having a garnet structure comprising at least Gd, Ce, Al, Ga and O, an atomic ratio of Gd/(Al+Ga+Gd) being 0.33-0.42, the amount of other phases other than the garnet structure being less than 2.0% by weight, the relative density being 99.0% or more, the diffuse transmittance being 50.0% or more, a main peak of the light emission spectrum being near 550 nm, and afterglow after 30 ms from turning off the exciting light being 10−3 or less. However, because the phosphorescent oxide of JP 2001-4753 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
JP 2003-119070 A discloses a fluorescent device formed by a matrix crystal with a garnet structure comprising Ce as a light-emitting element and at least Gd, Al, Ga and O, which has an absorption coefficient μ of 0.6 mm−1 or less to light having a wavelength of 550 nm, a relative density of 99.8% or more, and an average crystal grain size of 4 μm or more. However, because the phosphorescent oxide of JP 2003-119070 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
Japanese Patent 3938470 discloses a fluorescent body as a ceramic scintillator for a radiation detector, which has a composition comprising cerium as a light-emitting component and represented by the general formula of (Gd1-z-xLzCex)3Al5-yGayO12, wherein L represents La or Y, z is in a range of 0<z<0.2, x is in a range of 0.0005≦x≦0.015, and y is in a range of 0<y<5, and which is obtained by the compression-molding of starting material powders and sintering. Japanese Patent 3938470 describes that though part of gadolinium Gd may be substituted by yttrium Y, a larger amount of Y lowers the light emission efficiency and the X-ray stopping power, so that the amount (z) of Y is less than 0.2. Because the fluorescent body of Japanese Patent 3938470 contains Y in as small an amount as less than 0.2 by atomic ratio if any, it is not suitable for scintillators for detecting soft X-rays.
JP 2002-189080 A discloses a phosphorescent oxide with a garnet structure as a ceramic scintillator, which comprises at least Gd, Ce, Al, Ga and O, an atomic ratio of (Gd+Ce)/(Al+Ga+Gd+Ce) being more than 0.375 and 0.44 or less, an atomic ratio of Ce/(Ce+Gd) being 0.0005-0.02, and an atomic ratio of Ga/(Al+Ga) being more than 0 and less than 1.0. However, because the phosphorescent oxide of JP 2002-189080 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
JP 2001-303048 A discloses a fluorescent body as a ceramic scintillator, which is represented by the general formula of (Gd1-x-y-zLxCeyScz)3Al5-dGadO12, wherein L represents La or Y, x is in a range of 0≦x≦0.2, y is in a range of 0.0005≦y≦0.02, z is in a range of 0≦z≦0.03, and d is in a range of 0<d<5. JP 2001-303048 A describes that part of gadolinium Gd may be substituted by yttrium Y, a larger amount of Y lowers the light emission efficiency and the X-ray stopping power, so that the amount (x) of Y is less than 0.2. Because the fluorescent body of JP 2001-303048 A contains Y in as small an amount as less than 0.2 by atomic ratio if any, it is not suitable for scintillators for detecting soft X-rays.
JP 2001-294853 A discloses a phosphorescent oxide as a ceramic scintillator, which comprises Gd, Al, Ga and O as main elements, and Ce as a light-emitting component, 1 mol of the phosphorescent oxide containing more than 0.001 mol and less than 5.0 mol of a compound of at least one element of Groups Ia, IIa, IVb and VIIb in the Long Form Periodic Table. This phosphorescent oxide has a diffuse transmittance of 60.0% or more, X-ray sensitivity 1.8 times or more that of CdWO4, and afterglow of 5×10−5 or less after 300 ms from excitement. However, because the phosphorescent oxide of JP 2001-294853 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
JP 2001-183463 A discloses an oxide scintillator with a garnet-type crystal structure, which has a composition represented by the general formula of (Gd1-xCex)3Al5-yGayO12, wherein x is in a range of 0.0005≦x≦0.02, and y is in a range of 0<y<5, K or Si being less than 100 ppm by weight, and B being less than 100 ppm by weight. However, because the oxide scintillator of JP 2001-183463 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
JP 2007-145902 A discloses a phosphorescent oxide comprising Ce as a light-emitting element and at least Gd, Al, Ga and O, and mainly having a garnet-type matrix crystal structure, which further contains Na. However, because the phosphorescent oxide of JP 2007-145902 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
JP 2001-348273 A discloses a ceramic having a composition represented by the general formula of Gd3-xCexAlySizGa5-y-zO12, wherein 0.001≦x≦0.05, 1≦y≦4, and 0.0015≦z≦0.03. However, because the ceramic of JP 2001-348273 A contains Gd as a main component but does not contain Y, it is not suitable for scintillators for detecting soft X-rays.
WO 2006/068130 A discloses a phosphorescent material with a garnet structure comprising Ce as a light-emitting element and at least Gd, Al, Ga, O, and Lu and/or Y, and having a composition represented by the general formula(II) of (Gd1-x-zLxCez)3+a(Al1-uGau)5-aO12, wherein L is Lu and/or Y, 0<a≦0.15, 0<x<1.0 (0.2≦x≦0.67, when L is Y), 0.0003≦z≦0.0167, x+z<1.0, and 0.2≦u≦0.6. However, WO 2006/068130 A does not disclose the use of the phosphorescent material for scintillators for soft X-rays.
WO 2008/093869 A discloses a phosphorescent material with a garnet structure comprising Ce as a light-emitting element and at least Gd, Al, Ga, O, and Lu and/or Y, and having a composition represented by the general formula of (Gd1-x-zLxCez)3+a(Al1-uGau)5-aO12, wherein L is Lu and/or Y, 0<a≦0.15, 0<x<1.0, 0.0003≦z≦0.0167, x+z<1.0, and 0.2≦u≦0.6, Fe/Ce being 3% by weight or less. However, WO 2008/093869 A does not disclose the use of the phosphorescent material for scintillators for soft X-rays.
Demand is recently mounting to form a clear image of different-density tissues of a human body by X-ray CT. Among the human body tissues, low-density tissues such as muscles, blood vessels, etc. have large absorption coefficients of soft X-rays and small absorption coefficients of hard X-rays, while high-density tissues such as bones, etc. have large absorption coefficients for both soft X-rays and hard X-rays. Thus proposed is a method for imaging different-density tissues separately, using an X-ray detection device comprising scintillators 2 having a large absorption coefficient of soft X-rays and a small absorption coefficient of hard X-rays on an upper side, scintillators 3 having large absorption coefficients for both soft X-rays and hard X-rays on a lower side, silicon photodiodes 4 each arranged on the side of each scintillator 2, 3, light-reflecting members 5 covering both upper and side surfaces of each scintillator 2, 3, and a circuit board 6 below the scintillators 3 for hard X-rays as shown in FIG. 1 (JP 2008-538966 A). The upper-side scintillators 2 form an image of low-density tissues in a human body, and the lower-side scintillators 3 emit light by X-rays passing through the upper-side scintillators 2 to form an image of high-density tissues in a human body.
Known as scintillators having large absorption coefficients of soft X-rays and small absorption coefficients of hard X-rays are YAG:Ce and ZnSe(Te). However, YAG:Ce does not have a sufficiently large X-ray emission intensity, and ZnSe(Te) suffers extremely large unevenness of characteristics, toxicity, and expensiveness, despite excellent emission intensity and afterglow characteristics.